1. Field of the Invention
The present invention relates to a driving circuit for and a semiconductor device for driving a laser diode, which can switch between the on and off states of the laser diode at a high speed.
2. Description of Related Art
In addition to a driving current supply circuit for supplying a driving current to a laser diode when causing the laser diode to emit light, a related art driving circuit for driving the laser diode includes an auxiliary current supply circuit for, when the driving current rises, supplying an auxiliary current used for speeding up the rise of the driving current to the laser diode. The related art driving circuit further includes a current drawing circuit for drawing a current from the driving current when the driving current falls in order to speed up the fall of the driving current. Since the related art driving circuit thus has the auxiliary current supply circuit and the current drawing circuit, the related art driving circuit makes it possible to switch between the on and off states of the laser diode at a high speed.
In addition to the above-mentioned related art driving circuit, there has been provided a driving circuit for driving a laser diode, which controls the amplitude of a switching control wave so that the duty of the switching control wave becomes equal to 100% when a driving current supplied to the laser diode has a maximum amount, thereby reducing the power dissipation of a switching element, as disclosed by patent reference 1.
However, patent reference 1 discloses neither a technology for switching the on and off states of the laser diode at a high speed, nor technology for shaping the waveform of the driving current which is supplied to the laser diode.    [Patent reference 1] JP,5-218542,A (see paragraph numbers [0016] to [0024] and FIG. 3)
Although the former related art driving circuit that is so constructed as mentioned above can switch the on and off states of the laser diode at a high speed, a problem with the related art driving circuit is that since the driving current supplied to the laser diode decreases greatly immediately after the occurrence of an overshoot at the time of the rise of the driving current, whereas the driving current supplied to the laser diode increases greatly immediately after the occurrence of an undershoot at the time of the fall of the driving current, the driving current does not have a rectangular waveform and therefore a waveform shaping circuit for shaping the waveform of the driving current into a rectangular one is additionally needed.